1. Field of the Invention:
The invention relates in general to an apparatus for controlling gas flow for combustion in a two-stage furnace. More particularly, the invention relates to an improved valve for directly adjusting the gas flow in a two-stage furnace in response to the pressure drop across the heat exchanger system.
2. Background of the Prior Art:
A gas valve in a typical two-stage furnace includes two discreet position settings. Setting the gas valve to a first position results in a gas flow appropriate for supporting a low stage operating condition while setting the gas valve to a second setting results in a gas flow appropriate for supporting a high stage operating condition. The gas valve in general is switchable between a first and second position by actuation of a solenoid, which is energized via a pressure switch that is responsive to the pressure drop across the furnace heat exchanger system.
It has been the object of recent furnace control methods, however, to eliminate pressure switches which sense pressure drop across the furnace heat exchanger system. For example, application Ser. No. 08/602,436 assigned to a common assignee and incorporated herein by reference, describes a furnace control method which, in a dedicated system embodiment, does not require any pressure switches. In a method of the invention, a variable speed inducer motor in a two-stage furnace is controlled by a lookup table wherein torque required to sustain a stable excess air level (and constant heat exchanger pressure drop) is correlated with current motor speed. If low stage operation is required then the furnace operates in accordance with a low stage operating plot stored in the lookup table with the gas valve set to the first position. If high stage operation is required then the furnace operates in accordance with a high stage operating plot stored in the lookup table. When the high stage condition has been met, the gas valve solenoid is energized to change the gas valve setting from the first position to the second position.
There is a potential problem involved in the use of a two position gas valve in a furnace controlled according to a method wherein a gas valve is made responsive to lookup table. If, due to a selection error or other malfunction a lookup table for controlling a furnace inducer motor is selected that is inappropriate for the present furnace size, the present gas flow level may be inappropriate for the present combustion air level.